Monoclonal antibodies (MCAb) have been prepared using extracts of the rabbit T cell line RL-5. A total of 28 MCAb selected for positive reactivity with RL-5 and nonreactivity with the human cell line Daudi were tested by immunoprecipitation and surface fluorescence. The molecules precipitated by the MCAb fell into three major groups with molecular weights 120,000, 90,000 and 42,000. Reactivity of the MCAb with subpopulations of rabbit lymphoid cells is being investigated by FACS analysis. Attempts to prepare monoclonal antibodies directed against polymorphic determinants of HLA antigens resulted in an antibody that reacts with a number of lymphoid cell lines but with no normal cells, including peripheral blood lymphocytes taken from the donors of EBV transformed cell lines for which the antibody is strongly positive. The antibody precipitates a molecule with a two chain structure, the chains having molecular weights of 25,000 and 30,000 respectively.